This invention relates generally to improving the resistance of boxboard and paperboard to moisture absorption, and more particularly pertains to the application of a flexible film intermediate the surface of paperboard and the usual moisture retarding lacquer that is customarily applied thereto.
The paper and paperboard industry has long experimented and improved various methods for making paper products more resistant to the absorption of moisture, and this is particularly so where the materials are eventually folded into cartons and containers for use in the marketing of various foods, domestic and other chemical products. More specifically, the moisture retarding method and means for enhancing the attributes of paper usually entailed simply the coating of the moisture barrier lacquer directly to the paper product, generally incident to the initial manufacture of the paper, and if the paper was to be used in that particular initial configuration, then its moisture retarding attributes would generally be sufficient and beneficial during that usage. But, as is well known in the trade, such manufactured paper, particularly when it is further processed into packages or cartons, is almost without exception subsequently scored, or impressed with fold lines, and then folded by appropriate machinery into the configuration of cartons for support of enumerable products.
As an example of the foregoing, the patent to Hopermann, U.S. Pat. No. 3,770,486, discloses a method for preparing a moisture resistant packaging material which is generally formed by first applying a primer composition of a vinyl halide, vinylidene chloride, or a chlorinated rubber, to the surface of the material initially for sealing the pores of the packaging material to provide a pre moisture resistant coating. Then, after that coating is dried, a top lacquer coating of a cyclized natural rubber is applied to insure a reduction of moisture absorption into the paper. This prior art method is apparently effective for its intended operation, but, the applicant has found that when this prior art method is used on boxboard, subsequent bending or folding of the treated materials when folded into the configuration of a packaging material that significant fractures occur to the various coatings within the vicinity of the fold lines that substantially reduces, if not destroys, any barrier against moisture absorption of the finished container. It is believed that the inability of the identified primercoats to elongate is what causes the fractures to occur.
In fact, and further expanding upon the foregoing, tests have been conducted on packaging materials that have been treated according to the Hopermann method, as for example where Hopermann describes in Part C of its specification that the moisture vapor transmission rate had been subsided in his coated board down to a rate in the vicinity of 1.2 grams of water vapor per 100 square inches of treated board per 24 hours, that identical tests run upon the Hopermann board, that had been both scored and then folded, greatly accelerated its moisture absorbing characteristics as follows:
______________________________________ WATER VAPOR TRANSFER RATE-GRAMS/100 SQUARE INCHES/24 HOURS Hopermann Coating ______________________________________ Vertical Score Line Folded 180.degree. 35.9 Horizontal Score Line Folded Back 90.degree. Folded Forward 180.degree. 12.6 ______________________________________
Thus, it can be seen that if the prior arts original estimate, if actually proven, moisture vapor transmission rate was found to be 1.2 grams, then where the board was subsequently scored and then folded its moisture absorption was significantly increased by 2160% and 4170% respectively, as shown in each test instance. Hence, tests conducted on the Hopermann method having conclusively proven that its method is of little or no value where score lines and fold lines are subsequently made to the coated board.
Another prior art invention having some relevancy with respect to the present invention is in the U.S. patent to McDonald, U.S. Pat. No. 3,707,393, wherein is discussed the concept of adding a precoat or undercoat to the interior side of the paper, with said precoat comprising a co-polymer of ethylene and carboxylic acid. Then, a top coat of polyvinylidene chloride is also applied as an outside coating to the paper or sheet material. It is believed that the McDonald method for treating sheet material is probably used for the coating of paper, such as paper customarily used for holding food products such as potato chips, or other snack food products, and of the type that generally exhibit a form of grease barrier on the inside of a bag so as to prevent the oily film of the snack from penetrating to the exterior of the bag, and likewise to prevent any moisture from attaining entrance into the bag so as to maintain the crispness of the product contained therein. In any event, and for reasons which will be subsequently analyzed in the summary of the present invention, applicants invention utilizes a directly opposite method for coating boxboard products, in addition to one which significantly enhances the moisture absorbing barrier characteristic of such board.
It is therefore, the principal object of this invention to provide a precoating having significant elongation attributes, used as a precoating to the moisture barrier lacquer customarily applied as a coating to the boxboard and related paperboard products.
It is another object of this invention to provide a precoat for the standard moisture barrier lacquers applied to boxboard products, the purpose of the precoat being to provide resistance to fracture of said exterior lacquer during its folding or bending into the configuration of a carton.
It is a further object of this invention to provide a flexible precoat for the lacquer applied to paperboard products having the ability to stretch at least a 250 percent of its initial constitution and thereby avoid severing during the boxboard folding, or fracturing of the lacquer applied to the exterior surface.
A further object of this invention is to provide a flexible precoat having significant elongation attributes and which thereby helps to prevent any fracturing to the moisture barrier applied externally to paperboard.
Another object of this invention is to provide a precoat for boxboard having enhanced flexibility and which may be applied either before or after printed matter is gravured onto the board surface.
An additional object of this invention is to provide a flexible precoat for paperboard that will not be cut or fractured when fold lines are impressed onto its surface.
A further object of this invention is to provide a container formed from boxboard that has enhanced moisture retarding attributes due to the application of a flexible precoat initially to its surface.
A further object of this invention is to provide a vinyl resin precoat for boxboard which, after the application of a moisture barrier lacquer, reduces the water vapor transfer rate through the board, and particularly along its fold lines, by as much as one hundred times as compared to prior art lacquer coatings.
Another object of this invention is to provide an ionomer resin dispersion of an ethylene interpolymer as a precoating for boxboard to enhance the stretchability of any moisture barrier lacquer subsequently applied thereto before folding of the boxboard into the configuration of a carton.
These and other objects will become more apparent to those skilled in the art upon reviewing the following summary of this invention.